The present invention relates to electrical rotating machines, and more particularly to the stator of an electrical machine in which the rotor is located inside the stator.
The stator has a magnetic circuit and windings of electrically conductive wires which in general are made from insulated copper wire, often round in section. The magnetic circuit, for its part, is always laminated; it is formed by a stack of magnetic metal sheets. Each metal sheet is cut to shape such that slots separated by teeth are made, the slots being the seatings for the electrically conductive wires. Each slot is delimited by two substantially radially oriented walls and a slot base, and has an opening, the opening being located on a smaller radius than the radius on which the slot base is located. This principle of arranging the stator is widely applied in synchronous or asynchronous machines.
Installing the electric windings in the slots involves inserting electrical conductors (or sections of electrical conductors) by passing them through the opening of the slot. Consider that, in the type of motor relevant here, this slot opening is oriented towards the inside and is thus hard to access. Moreover, the slot opening is in general quite narrow with respect to the width of the slot. In fact, to arrive at the optimum mechanical and magnetic construction, each tooth ends (on the side with the small radii) in a foot which partly closes the slot opening.
It is thus no simple matter to insert the electrical conductors in the slots by passing them through this rather cramped opening. This results in filling rates of the slots with the electrical conductors remaining relatively low, in particular if the electrical machine is long.
The dimensions of an electrical rotating machine depend on its rated load torque. The higher the rated load torque which a motor is able to produce, the more bulky the electrical motor, all other things being equal.
There are applications for which it is desirable to obtain both high power levels and highly compact constructions of the motor at the same time. To give just one concrete example, when the intention is to install electric traction motors in the wheels of automotive vehicles, it is desirable to be able to develop power levels of at least 10 kW per motor, and even at least 25 or 30 kW per motor for the majority of the time, for a weight as low as possible in order not to make the unsprung masses too heavy. It is also desirable for the bulk to be very small, not going beyond or going beyond by as little as possible the internal volume of the wheel so that it does not interfere with the elements of the vehicle in the event of flexing of the suspension and in the event of other types of movement of the wheel with respect to the vehicle body.
These two requirements (high power level and low weight and bulk) make it very problematic to install electric traction motors in the wheels of private vehicles without radically improving the ratio of weight to power of the electrical machines currently available on the market.
The object of the invention is to facilitate and optimize the construction of electric motors. This is most particularly important if the intention is to increase the specific power of an electrical rotating machine, that is to say the mechanical power that a motor is capable of developing for a given overall mass or the electrical power that an alternator is capable of delivering for a given overall mass.
The invention relates to electrical machines in which the stator magnetic circuit is made in two parts: an internal part comprising the teeth, and an external part covering the teeth and slots. In this magnetic circuit, each tooth separating two slots does not form a continuous piece with the outside periphery of the magnetic circuit. It goes without saying that the magnetic circuit is laminated and is made from ferromagnetic metal sheets for reasons well known to those skilled in the art. It is thus possible to form a core comprising the internal part of the magnetic circuit, on which the conductive wires can be wound in the slots, with access to these slots being from the outside. Then, the internal part is covered by the external part to complete the magnetic circuit.
In a first aspect, the invention proposes an electrical rotating machine comprising an external stator and a rotor having a geometrical axis of rotation and disposed inside the stator, the said stator comprising a laminated magnetic circuit having:
a stack of yokes each made from a magnetic metal sheet and disposed substantially parallel to a plane perpendicular to the axis, the stack forming an external covering;
a plurality of teeth disposed inside the covering, protruding towards the inside, the teeth each being made from a magnetic metal sheet and disposed substantially parallel to a plane perpendicular to the axis, the teeth being stacked and the stacks of teeth delimiting slots, the teeth being bound to the covering;
the slots being delimited on the radially outer side by the yokes, the slots being delimited in the circumferential direction by the side walls of the teeth, electrically conductive wires being disposed in the slots, and in which the covering is mounted such that it is bound to a sheath surrounding the covering on the outside thereof, a circuit intended for a cooling liquid being made in the said sheath.
One object of the invention is at the same time to improve dissipation to the outside of the heat generated by the motor. The invention proposes using a cooling circuit by means of ducting in which a liquid coolant circulates, which, despite the thermal resistance owing to the stator magnetic circuit in two parts, which allows better filling of the slots by the copper, enables the heating of the motor to be sufficiently limited and thus allows good efficiency and/or an increased specific power to be achieved.
To construct machines of very high power per unit mass, and/or to construct machines with very good efficiency, it is advantageous to fill the available section of the slot with copper to the maximum extent. With a given slot section, an increase in the section of copper brings about a decrease in Joule""s loss for a given current (and thus torque). Moreover, there is less heating. The efficiency of the machine is thus improved. In accordance with a different approach, at a given maximum permissible temperature, the corresponding current is larger (and thus the torque is greater), which improves the ratio of weight to power of the machine.
Consider also that for a given section of copper the section of the slot can be reduced and thus the mass of the ferromagnetic circuit is reduced. In fact, all other things being equal, in particular the maximum permissible temperature within the machine, it is possible for example to reduce the height (dimension measured in the radial direction) of the slots. The mass of the ferromagnetic circuit has been reduced, the losses in the ferromagnetic circuit are thus decreased, and consequently the specific power of the machine and its efficiency are improved.
According to a second aspect, the invention proposes an electrical rotating machine comprising an external stator and a rotor having a geometrical axis of rotation and disposed inside the stator, the said stator comprising a laminated magnetic circuit having:
a stack of yokes each made from a magnetic metal sheet and disposed substantially parallel to a plane perpendicular to the axis, the stack forming an external covering;
a plurality of teeth disposed inside the covering, protruding towards the inside, the teeth each being made from a magnetic metal sheet and disposed substantially parallel to a plane perpendicular to the axis, the teeth being stacked and the stacks of teeth delimiting slots, the teeth being bound to the covering;
the slots being delimited on the radially outer side by the yokes, the slots being delimited in the circumferential direction by the side walls of the teeth, electrically conductive wires being disposed in the slots, and having a stack of star-shaped metal sheets each made from a magnetic metal sheet and disposed substantially parallel to a plane perpendicular to the axis, all the teeth appearing on each star-shaped metal sheet, all the teeth being connected to one another by a partition closing the slot on the radially inner side of the slot.
According to this second aspect of the invention, it is proposed that a motor be constructed in such manner that the slots are not open towards the air gap separating the rotor from the stator. The torque created in the motor is the result of the interaction between the flux created at the rotor (by the magnets or the electromagnets in the case of a synchronous machine) and the flux created at the stator by the circulation of current in the windings of the stator. The lines of flux coming from the rotor must encompass the flux created at the stator. It is commonly recognised that this result is obtained because the base of the slots of the stator is open towards the axis of the electrical machine, which prevents a magnetic short-circuit for the flux coming from the magnets or coming from the windings of the stator. Moreover, to improve the mechanical behaviour of the copper in the slots, it is conventional to provide feet on the slots. These must be sufficiently thick, typically in the order of 0.7 to 0.9 mm, or else the feet on the slots will be too fragile to contain the copper wires inside the slot.
Contrary to the usual design, in this aspect of the invention, the feet on the slots are closed by the magnetic circuit over the entire inside periphery of the stator. A very thin partition on the radially inner side of the slot, for example less than 0.5 mm thick, and preferably less than 0.4 mm thick, is sufficient to give it a very high level of mechanical strength, because the partition is continuous. Because of its thinness, a very high level of magnetic saturation is obtained. The result is that the leakage flux passing across this partition remains very small and has no noticeable effect on the motor torque. Moreover, because the partition is thinner at the foot on the slot than in conventional arrangements for slot feet of open slots, the result is a gain in the slot section available for installing the copper. It is thus possible to install more copper, all other things being equal.
As already explained, to construct machines of very high power per unit mass, it is advantageous to fill the available section of the slot with copper to the maximum extent, and this is facilitated by the invention. All other things being equal, in particular the level of saturation in the magnetic circuit, the height of the slots can be reduced.
The invention will be better understood from the non-restrictive description which follows and which is given with reference to the attached drawings.